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ISOTHERM STUDIES OF EQUILIBRIUM SORPTION OF

Fe(III) ONTO UNMODIFIED HEN EGGSHELL ADSORBENT

Dr. Yogesh C. Rotliwala1, Mikesh H. Chevli

2, Neha Maheshwari

3

Head, Chemical Engineering, SNPIT & RC, Bardoli, Gujarat, India 1

Assistant Professor, Chemical Engineering SNPIT & RC, Bardoli, Gujarat, India 2

Student, Energy & Environment, Symbiosis Institute of International Business, Pune,

Maharashtra3

Abstract: The paper explores the study of equilibrium isotherms and potentiality of

untreated hen egg shell as a bio waste sorbent for the removal of Fe (III) from aqueous

solution. The adsorption data were fitted to different Isotherm models. Regression

coefficient (R2) for different models was calculated and observed to be in the order of

Langmuir > Freundlich > Temkin > Dubinin–Radushkevich. Favourable adsorption was

analysed by determining separation factor (RL=0.136) in a Langmuir isotherm and cross

verified by adsorption intensity (n=1.61) in a Freundlich isotherm. Moreover, physical

adsorption was confirmed with the apparent energy of adsorption (1.0 kJ/mol) in a

Dubinin-Radushkevich isotherm model and heat of adsorption (8.239 J/mol) from

Tempkin isotherm model. Study shows that untreated hen egg shell can be utilised as a bio

waste sorbent for the removal of Fe (III) from aqueous solution.

Keywords: Equilibrium adsorption modelling, Heavy metals, Hen egg shell, Isotherm,

Sorption

I. INTRODUCTION

Due to industrialization in last two decades, there has been growing concern over

the diverse effects of heavy metals bearing waste water on humans and aquatic

ecosystems [1]. Major source of heavy metal bearing waster water mainly includes steel

industries, aeronautic, metal coating, etc and requires costlier treatment, e.g. ion

exchange, biodegradation, oxidation, solvent extraction and adsorption [2, 3]. Recently,

adsorption technique (with the use of waste solids) for the removal of heavy metals from

the effluent is reported effective due to low cost, ease of handling and less chemical

consumption. Moreover, resource recovery potential for the value added metal can be

exploited [4, 5].

Food processing, egg breaking and hatching industries are the major source of egg

shell waste (11 % of egg weight), which are available in huge quantities. About 2,50,000

tons of egg shell waste is produced annually worldwide by the food processing industry

only. Most of the eggshell waste is commonly discarded of in landfills without any

pretreatment because it is traditionally useless and ultimately creates serious

environmental problems [6]. The chemical composition of egg shell (94% calcium

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carbonate, 1% magnesium carbonate, 1% calcium phosphate and 4% organic

material), as well as the porous nature of egg shell structure (7000-17000 pores) [7]

leading to serve as attractive material as an adsorbent. Recovery and utilization of

calcium in egg shell for the production of cellulose is reported [6]. Chicken egg shell

membrane accounted to be effective low cost adsorbent for the removal of anion and

cation dyes [8]. Many authors have developed costlier method for the removal of iron

from waste water with the use of duck and crab egg shell [9, 10, 11].

The present work addresses the study for effect of adsorbent (eggshell) and

adsorbate (stock solution) on the adsorption efficiency of Fe(III) from synthetic aqueous

solution by hen eggshell. Besides, the present study is also aimed to evaluate the

equilibrium of adsorption process using Langmuir, Freundlich, Temkin and Dubinin–

Radushkevich (D – R) Isotherms.

II. MATERIALS AND METHODS

A. Preparation of Eggshell

The hen eggshell used in this study was obtained from local food stall located in the city of

Surat, India. The eggshell was washed with distilled water several times to remove surface

impurities, sundried followed by oven dried at 105 °C for 2 hours. The dried eggshell was

grounded using pestle and mortar and the particle size was adjusted in the range of 125-500

µm to increase the surface area for efficient adsorption. Physico-chemical properties of hen

eggshell are shown in Table 1.

TABLE 1:- PHYSICO-CHEMICAL CHARACTERISTICS OF HEN EGGSHELL

Surface Area 7.0 m2/g Saers Method (Reference 12)

pH 8.1-8.9 IS 3025 (Part 11) (1983, Reaffirmed 2002)

Moisture 8 % ASTM D5142- 02a

Particle Size 125-500 µm Using Sieve Analysis

Bulk Density 1.346 g/cm3 S. Toshiguki et al. (Reference 13)

B. Preparation of Adsorbate

The adsorbate was prepared by synthesizing standard stock solution of Fe(III) by dissolving

in distilled water. All chemical used in the present study were of analytical grade from Fisher

Scientific, India. Atomic Absorption Spectrophotometer (AAS Model A 303, Thermo

Scientific) was used to measure the Fe(III) concentration.

C. Experimental

Series of solutions containing different initial concentrations of Fe(III) ions (in the range of 5-

50 mg/L) was prepared and employed for the batch adsorption studies at 30°C to check the

applicability of the Langmuir, Freundlich, Temkin and Dubinin– Radushkevich (D – R)

adsorption isotherms under optimum conditions (120 minutes on the magnetic stirrer at

constant revolution in the pH range between 6 to 6.5). The mixture was centrifuged and the

iron concentration was determined using Atomic Absorption Spectrophotometer (AAS Model

A 303, Thermo Scientific). As shown in Table 2 the amount of solute adsorbed and the

removal efficiency was calculated with the use of reported mass balance equations [14].

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……………. (1)

………… (2)

Where, Q is the amount of solute adsorbed from the solution (mg/L). V is the volume of the

adsorbate (L), Ci is the concentration before adsorption (mg/L), Cf is the concentration after

adsorption (mg/L), M is the mass of eggshell used (g) and E is the percent removal

efficiency.

III. RESULTS AND DISCUSSION

A. LANGMUIR ADSORPTION MODEL

Monolayer adsorption process is described quantitatively by Langmuir adsorption model. The

model assumes uniform contact between the surface of adsorbate and adsorbent and that;

there should be specific number of adsorption sites on the surface of the adsorbent onto

which the solute molecules can be adsorbed [15]. Further, the energy of adsorption onto the

surface should be uniform and there should be no transmigration of adsorbate in the plane of

the surface [16]. Based upon these assumptions, the Langmuir adsorption model can be

represented as under,

……………. (3)

Adsorption parameters were evaluated following the linear form of equation 3,

………….. (4)

Where, qmax is the maximum monolayer capacity of adsorbent (mg/g), b is the Langmuir

constant (L/mg), Ce is the concentration of adsorbate under equilibrium conditions (mg/L)

and qe is the amount of adsorbate per gram of adsorbent at equilibrium (mg/g).

The value of qmax and b was obtained from the slope and intercept of the plot of 1/qe versus

1/Ce as shown in Fig 1. The results of Langmuir plot are shown in Table 3.

An important characteristic of the Langmuir isotherm may be expressed in terms of

dimensionless equilibrium parameter constant RL [17] expressed as,

…………… (5)

Where, Co is the initial adsorbate concentration and b is the Langmuir isotherm constant.

According to Hameed et al., 2007, Bayramoglu et al., 2009 and Ghanizdeh et al., 2010, the

value of RL gives a good approximation of adsorption characteristics, the nature of adsorption

is unfavorable if RL>1, linear if RL =1, favorable if 0< RL<1 and irreversible if RL=0. From

the values computed in Table 3, it can be seen that the value of RL lies between 0 and 1

indicating that the adsorption experiment considering the Langmuir isotherm model is

favorable under the specified experimental conditions of this research work. Also, from the

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results it can be noted that the coefficient of regression value, R2 is 0.926 which signifies that

the isotherm model gives good and best fit with the experimental adsorption data under

specified experimental conditions.

B. FREUNDLICH ADSORPTION ISOTHERM

The Freundlich adsorption model normally applies for the heterogeneous surface. The

isotherm model was plotted to study the intensity of adsorption of adsorbent for the adsorbate

[21]. The linear form of Freundlich isotherm equation is expressed as,

…………. (6)

Where, Kf is the Freundlich isotherm constant signifies the capacity of adsorption process

(mg/g), while n gives good approximation of the adsorption intensity. Equation 6 was plotted

against logqe versus logCe as shown in Fig 2 to get the values of n and Kf from the slope and

intercept respectively. The adsorption data shows a reasonable fit with Freundlich isotherm

model with R2

value of 0.884. A favorable adsorption process is estimated when the value of

n lies between one and ten [22]. From the calculated results, it can be seen that the value of n

is 1.61 indicating a favorable adsorption process. On the other hand, 1/n is anticipated as a

function of the adsorption strength in the adsorption process [23]. Specifically, the linear

least-squares method and the linearly transformed equations have been widely applied to

correlate sorption data where 1/n is a heterogeneity parameter, the smaller 1/n, the greater the

expected heterogeneity [16]. If value of 1/n is below one it indicates a normal adsorption

process, while 1/n being above one indicates cooperative adsorption [24]. From Table 3, the

value of 1/n is 0.619 indicating a normal adsorption process. It can be seen from equation 6

that the amount of adsorption varies linearly with pressure until the specific saturation

pressure is reached after that no further adsorption is possible and the system attains

equilibrium.

C. TEMPKIN ISOTHERM MODEL

The Tempkin isotherm model represents a linear relationship between the amount of

adsorbate per gram of adsorbent at equilibrium and lnCe. Moreover, the adsorption model

assumes that the heat of adsorption of each molecule decreases linearly with the coverage

capacity by ignoring the extremities of concentration [25, 26]. The adsorption model is

represented by following linear equation [25],

………….. (7)

……………………………….. (8)

Where, A represents the equilibrium Tempkin isotherm binding constant (L/g), R is the

universal gas constant (8.314 J/mol/K), bT is Tempkin constant, T is room temperature in K

and B is the constant of heat of adsorption which usually express the type of adsorption

(J/mol).

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The linear equation 7 was plotted against qe versus lnCe to get the values of B and A as the

slope and intercept respectively and the value of bT was calculated from equation 8. Fig 3

shows scattered nature of the graph with R2

value as 0.678 indicating less fit of Tempkin

isotherm with the experimental adsorption data. From Table 3, the value of heat of adsorption

is 8.239 J/mol indicating a physical adsorption process.

D. DUBININ–RADUSHKEVICH ISOTHERM MODEL

The adsorption mechanism considering the distribution of Gaussian energy onto

heterogeneous surface is normally represented by Dubinin–Radushkevich isotherm model

[27]. The model is represented by linear equation 9.

…………… (9)

Where, qs is termed as the theoretical value of isotherm adsorption capacity (mg/g), Kad is

Dubinin–Radushkevich isotherm constant (mol2/KJ

2) and ε is Dubinin–Radushkevich

isotherm constant. The graph of lnqe versus ε2 considered as the DRK isotherm plot yields the

value of logarithm of theoretical adsorption capacity lnqs as the intercept and DRK constant

Kad as the slope of the characteristics curve. The apparent energy of adsorption (E) which is a

measure of the characteristics of adsorption process is calculated from equation 10.

………….. (10)

Where BDR is expressed as the isotherm constant, whereas, the parameter ε can be calculated

by the equation 11,

…………… (11)

Where, R is gas constant (8.314 J/mol K), T is the absolute temperature (K) and Ce is the

adsorbate equilibrium concentration (mg/L). The DRK isotherm parameters are tabulated as

per Table 3. As can be seen from Fig 4 this isotherm model also does not provide a good fit

with the adsorption data and the R2 value is 0.545. P. Sivakumar et al. found that if the value

of E lies between 8 and 16 kJ/mol then it indicates a chemisorptions process, whereas, if the

value lies below 8 kJ/mol then it indicates a physical adsorption process. Here from the DRK

plot the apparent energy of adsorption (E) value is obtained as 1 kJ/mol indicates physical

adsorption process between unmodified eggshell and Fe(III) ions.

Table 3:- Langmuir, Freundlich, Temkin and Dubinin-Raduskevich isotherm constants

for the adsorption of Fe(III) onto unmodified hen egg shell

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IV. CONCLUSION

The experimental result showed the use of unmodified hen eggshell as a low cost adsorbent

for Fe(III) bearing aqueous solution. The adsorption isotherm observed in the range of

Langmuir > Freundlich > Temkin > Dubinin–Radushkevich. Physical adsorption was

confirmed with the apparent energy of adsorption (1.0 kJ/mol) in a Dubinin-Radushkevich

isotherm and heat of adsorption (8.239 J/mol) from Tempkin isotherm model. Favourable

adsorption was analyzed from separation factor (RL=0.136) in a Langmuir isotherm and cross

verified by adsorption intensity (n=1.61) in a Freundlich isotherm. Hence, hen egg shell can

be utilised as a low cost bio waste sorbent for the removal of Fe(III) from aqueous solution

without chemical treatment as this will increase the cost of application.

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ACKNOWLEDGMENT

The authors would like to thank professors and lab assistants of Chemical Engineering

Department and Environmental Audit Cell of S. N. Patel Institute of Technology & Research

Centre, Umrakh, Bardoli for their extended motivational and infrastructure support in

carrying out this research work.

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